A new higher speed limit for lorries, promoted by ministers for its safety benefits, is more likely to lead to a 10% rise in fatal accidents, the government's own impact assessment shows.

After the decision to let HGVs drive at 50mph instead of 40mph on single-carriageway rural roads was announced, the new transport minister, Claire Perry, claimed the move would "cut dangerous overtaking". The announcement – slipped out at the start of the Commons recess – was backed by industry groups including the Road Haulage Association, which was quoted by the government as saying: "This evidence-based decision by ministers will be strongly welcomed by hauliers and their drivers. The current limit is long out-of-date and the frustration it generates causes unnecessary road safety risks."

Cyclists and road safety campaigners, however, have criticised the decision, and an internal impact assessment conducted by the Department for Transport shows the death toll would be expected to rise, with two to three extra fatal accidents a year caused by lorries going faster as well as four to nine extra serious accidents – a 10-20% rise in casualties.

While coalition ministers said safety would be improved by limiting dangerous overtaking, the assessment said it was a potential benefit but not quantified "because we do not have sufficient confidence that it would occur". It added that, "while overtaking manoeuvres may become less likely, they would also be performed at higher speeds and so could become more dangerous".

The DfT assessment also shows that 75% of HGVs are believed to routinely break the speed limit when away from speed cameras.

Christopher Peck, policy coordinator of CTC, the national cycling charity, said: "The risk of cycling on rural single-carriageway roads is over 20 times greater than on minor urban roads, and several cyclists are killed each year – hit behind by lorries on these roads – a risk which will only increase as lorries are allowed to go faster. CTC believes that lorries should only be allowed to drive at higher speeds on properly engineered major roads, where adequate parallel cycling facilities exist."

A DfT spokesman said: "Road safety is a key priority and we studied both the potential for increased risk and for improved safety due to less risky overtaking before making our decision. We are determined to improve safety – for instance, by encouraging local authorities to lower speed limits on roads where needed, better procedures to deal with HGV drivers who drive tired, and bringing in a new offence of causing serious injury by dangerous driving."

Shadow transport secretary Mary Creagh accused ministers of sneaking out the announcement. "The government has pledged to review the safety of rural roads, but these higher speed limits will make them much less safe for pedestrians and cyclists. Ministers need to bring forward evidence before pushing ahead with these potentially dangerous speed increases."​

Raising speed limits has long been a target of the freight transport industry. Perry said the change would "get Britain moving and boost growth". The government says it will save the haulage industry £11m a year, and points to a £3.3bn investment in major road schemes on the strategic road network as well as £10.7bn earmarked for motorways.

However, Labour said the pledged money had only offset some of the early coalition cuts to infrastructure plans and pointed to a shortfall of more than £150m in spending on road maintenance over the parliament.

Creagh said: "The main impact on the freight industry is that the government has failed to tackle the strategic road network, cancelling projects to improve roads and cutting building and maintenance budgets."

The speed limit changes on rural roads will be made law in the autumn to come into effect early next year. A parallel proposal to raise HGV speed limits from 50mph to 60mph on dual carriageways is under consultation until September but is likely to come into force at the same time.

A report by the Campaign for Better Transport last month concluded that HGVs were effectively subsidised to the tune of £5bn by the taxpayer, with road tax covering only a fraction of their external damage. The heaviest lorries are responsible for the majority of wear and tear on road surfaces, and HGVs are five times more likely than cars to be involved in fatal accidents on local roads.